This invention relates to the field of ceramics, and particularly to Si.sub.3 N.sub.4 ceramics.
Silicon nitride ceramics encompass a family of compositions in which Si.sub.3 N.sub.4 constitutes an important ingredient. Considerable effort has been expended and is being expended to develop ceramics which have useful structural properties at high temperatures. In particular, there is a continuing need for Si.sub.3 N.sub.4 ceramics which have high oxidation and creep resistance.
Si.sub.3 N.sub.4 ceramics are produced by compacting Si.sub.3 N.sub.4 powder and then sintering or hot pressing the powder at high temperature. Very pure Si.sub.3 N.sub.4 powder cannot be successfully sintered or hot pressed and consequently a densification aid must be added to the Si.sub.3 N.sub.4 powder in order to obtain dense Si.sub.3 N.sub.4. The densification aid reacts with SiO.sub.2 on the surface of the Si.sub.3 N.sub.4 particles, with the Si.sub.3 N.sub.4, and with impurities in the powder to form a eutectic liquid during sintering. The liquid promotes densification of the powder through a solution-reprecipitation mechanism.
Upon cooling, the liquid freezes and forms secondary phases. The number, chemistry, and content of the secondary phases depend on the constituents in the starting powder and the phase relationship of the composite system. Both non-equilibrium (nitrogen glasses) and crystalline phases are observed. Thus, Si.sub.3 N.sub.4 alloys are polyphase materials. As expected, the secondary phases significantly influence the properties of the ceramic, unfortunately by decreasing the ceramic's creep and oxidation strength at high temperatures.
One approach to overcoming the harmful effect of the secondary phase is to reduce the amount of secondary phase in the ceramic. U.S. patent application Ser. No. 266,244, filed May 22, 1981 by F. F. Lange and D. R. Clarke, describe a method of drawing out some of the glassy phase by a post-fabrication oxidation treatment.
Another approach is to use ingredients which produce a secondary phase which has improved high temperature properties. This second approach is attempted in the method described in U.S. Pat. No. 4,046,580, wherein Y.sub.2 O.sub.3 is used as a densification aid.
Currently, MgO is the densification aid used in most commercial Si.sub.3 N.sub.4 ceramics. Other known aids include ZrO.sub.2, Al.sub.2 O.sub.3, BeO, and the rare earth oxides Y.sub.2 O.sub.3, CeO.sub.2, Ce.sub.2 O.sub.3, Nd.sub.2 O.sub.3, and La.sub.2 O.sub.3. Unfortunately, all these aids form intergranular phases which are either viscous glasses at high temperature (thus reducing the ceramic's creep strength) or are metal-silicon-oxynitride compounds (sometimes causing cracking by expanding during oxidation).